The present invention relates to cardial pacing systems, and, in particular, to cardial pacing systems for detecting low amplitude noise and determining a representative noise floor in a cardiac pacing system.
Implantable cardiac pacemakers must accurately process sensed signal information so as to determine when a genuine cardiac signal has, in fact, been sensed. Furthermore, implantable cardiac pacemakers must also accurately identify or classify the signal. Separating cardiac signals from polarization effects and other noise artifacts has always been a substantial problem, and a great deal of effort has been placed on improving input circuits for this purpose. Additionally, it is recognized that it is important to be able to classify a sensed signal, e.g., determine whether it is a QRS, P-wave, far field R-wave (FFRW), etc. Many prior art techniques have been developed for signal classification, but improvement is still needed. For example, one prior art technique establishes a variable timing window, and classifies the event in terms of the timing of the signal received during window. However, early beats; ectopic signals, etc. can fool such a technique, and noise can still mask the signal which is sensed within the window. Other known techniques include morphology analysis, comparisons in the time and frequency domain, etc. While many of these techniques provide reasonably good results, they can involve considerable circuit complexity and frequently do not eliminate the probability of error due to the detection of noise or other artifacts.
The advent of digital signal processing (DSP) has provided a tool which can be very useful in the environment of an implanted medical device, e.g., an implanted pacemaker. In DSP technology, the incoming sense signal is converted to a digital signal, e.g., an 8 bit signal at a predetermined sample rate. Successive digital signals can be processed with high reliability and in a manner which is essentially hardware-controlled by the DSP circuitry. More recently, DSP technology has advanced so as to provide the possibility of a low current chip which may be used in an implantable pacemaker to provide significant sensed signal processing capability.
The utilization of a DSP chip for an implantable pacemaker makes available an enhanced capability of processing sensed signals, so as to enable a more accurate classification of a sensed signal. Such DSP processing, together with a processor and an appropriate signal classification algorithm, can provide a powerful tool for accurately sensing and classifying intracardiac signals. In addition to this combined hardware and software capability, there is a need to provide an optimum decision algorithm for using the DSP-generated signal parameters so as to accurately and reliably classify sensed intracardiac signals.
The invention disclosed in U.S. Pat. No. 6,029,087, issued to Wohlgemuth, discloses a DSP solution for sensing a sensed event classification. Wohlgemuth presents a method which includes steps for digitizing a cardiac signal using low power ADC; digitally filtering and calculating a slope; determining a sense when both the filtered signal and the slope are above a predetermined threshold; finding local minimum and maximum points on both the filtered signal and the slope signal during a window period which follows the sense period; and classifying an event by timing the minimum and maximum points during that window.
The invention disclosed in U.S. Pat. No. 5,755,738, issued to Kim et al, discloses an automatic sensing level adjustment means for implantable cardiac rhythm management. In Kim, a sense threshold is set at a fraction of an average peak amplitude of two preceding events. An event threshold is then set at a smaller fraction of the sense threshold. If an additional event exceeds both thresholds, a cardiac event is then differentiated from the noise.
In U.S. Pat. No. 5,564,430, issued to Jacobson et al, a sensing threshold is set at a predetermined fraction of a measured sensed signal. If a further signal is sensed during a xe2x80x9cnoise refractory interval,xe2x80x9d the refractory interval is restarted and the sense threshold is incremented.
Finally, in U.S. Pat. Nos. 5,103,819 and 4,880,004, both issued to Baker et al, an invention is disclosed which describes a marathon auto sensing automatic sensitivity adjustment. This sensitivity adjustment utilizes an automated gain control device to control the signal amplitude between a minimum and maximum threshold.
As discussed above, the most pertinent prior art patents are:
All patents listed in Table 1 are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, the Detailed Description of the Preferred Embodiments and the Claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the teachings of the present invention.
The present invention is therefore directed to providing a method for determining a noise floor in a cardiac pacing system. Such a system of the present invention overcomes the problems, disadvantages and limitations of the prior art described above, and provides a more efficient and accurate means of both detecting and determining a noise floor.
The present invention has certain objects. That is, various embodiments of the present invention provide solutions to one or more problems existing in the prior art respecting the determination of a noise floor. Those problems include, without limitation: receiving a signal corresponding to a level of background noise, determining whether the absolute value of the received level of background noise is greater than a previously recorded version of background noise, and setting a noise floor when an appropriate number of satisfactory measurements have been taken.
In comparison to known techniques for stimulating the contraction of an atrium and/or ventricle of a mammalian heart, various embodiments of the present invention may provide one or more of the following advantages: receiving a signal corresponding to a level of background noise, determining whether the absolute value of the received level of background noise is greater than a previously recorded version of background noise, and setting a noise floor when an appropriate number of satisfactory measurements have been taken.
Some of the embodiments of the present invention include one or more of the following features: an implantable medical device including a processor, a controller operably connected to the processor and at least one sensing lead operably connected to the controller, wherein a noise floor is determined by the processor as an absolute maximum value of a plurality of recorded noise levels received by the processor from the at least one sensing lead, each of the plurality of recorded noise levels being below a percentage.
Furthermore, in accordance with the present invention a method for determining a representative noise floor in a cardiac pacing system from which future events are measured is provided. An event amplitude is determined. Pluralities of noise levels are recorded, when each of the plurality of noise levels is less than a predetermined percentage of the event amplitude. The noise floor is determined as an absolute maximum value of the plurality of recorded noise levels.
Therefore, the algorithm of the present invention enables the implantable medical device to detect and determine a noise floor by measuring the background noise of a mammalian heart and setting the absolute value of that background noise to be a representative noise floor. In this way it is possible to more accurately detect sensed events within the mammalian heart while not being distracted from mirror background noise.